In known circuits, the back-up battery is commonly coupled to the power supply via simple isolation diodes or series transistor switches. Both of these switching techniques are relatively inefficient and the use of isolation diodes is not practical at operating voltages below 2 volts. In a 1 volt application, such as a watch pager, the back-up cell or back-up capacitor will be connected to the entire system that was previously powered by the primary battery. If a power draining function such as an audio alert, lamp, or vibrator is being performed when the primary battery is removed, then the back-up power will quickly be depleted from the back-up cell or capacitor. The depletion can occur so quickly, that perhaps even a controlling microprocessor would be unable to turn off the power depleting portion of a circuit in time to prevent such a power drain. Additionally, using an integrated circuit or a microswitch capable of providing isolation of the back-up cell or back-up capacitor would involve more unnecessary parts and cost to such a product. Thus, there exists a need for a fail safe mechanism in a circuit to protect against the back-up cell or capacitor from sourcing current to a power draining portion of a circuit such as the alert, lamp or vibrator when the primary battery is removed from the circuit.